For problems related to the HTML 5 media elements (<video> and <audio>) -- including WebM, MP4, MSE and EME issues. This would also typically include decoding problems in the codecs themselves (e.g. VP8, VP9, H.264, AAC) experienced during playback.

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User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:37.0) Gecko/20100101 Firefox/37.0
Build ID: 2015040300
Steps to reproduce:
Just try to play any mp3 audio file in native HTML5 player.
This stopped working on my Linux box (openSuSE 12.3) after upgrading from 36.0.4 to 37.0.1. Downgrading to an earlier FF version makes it work again.
Actual results:
Error message saying that file is damaged, but it isn't.
Playing the same audio file from FF v37.0.1 on Windows 7 works alright.

Works for me in Firefox 37.0.1 in Linux Mint KDE x64 17.1.
1. http://hpr.dogphilosophy.net/test/
2. Next to the MP3 entry, click “(click to try it)”
Can you play the above MP3 file? If not, please provide a link to the MP3 file that failed to play. Either way, please provide the full and exact error message displayed.

Thanks, Gingerbread.
Of the audio files from the above link most do play, but the mp3 and the flac file formats don't.
In either case I get an alert saying: Like, DUDE! Something went wrong!
Miscellaneous problem with media data.
The problem with FLAC is an issue with plugins that could be fixed. I think that's not relevant here. But MP3 should work without extra plugin (or so I think).
For the MP3 the console says:
GET http://hpr.dogphilosophy.net/test/mp3.mp3 [HTTP/1.1 206 Partial Content 240ms]
Medien-Ressource http://hpr.dogphilosophy.net/test/mp3.mp3 konnte nicht dekodiert werden.
And opening the MP3 in its own tab produces this error message: Video kann nicht abgespielt werden, weil die Datei beschädigt ist.
Those messages are the same I got on the original MP3 that caused me to write the bug report.

Thanks again.
GStreamer is installed on my system in both versions 1.0.5 and 0.10 plus a whole bunch of related libraries and GTK plugins. No GStreamer plugin is explicitly listed in firefox.
Trying to play any of the MP3 files we talked about with a brand new profile in FFv37.0.1 fails identically as with my standard profile.
Trying to play any of those MP3 files in the nightly build (firefox-40.0a1.en-US.linux-x86_64) with a brand new profile fails likewise, now the message being "video can't be played because the file is corrupt", which means the same thing as the message in the German localization, so it's not a localization issue.
> FLAC isn't supported. So FLAC not playing isn't a bug, unless you have a plug-in that's supposed to
> play that format. And even then, it might be a bug in the plug-in. Either way, it would be the topic
> of a separate bug report.
Yeah, let's forget about the FLAC.

(In reply to wf212 from comment #4)
> No GStreamer plugin is explicitly listed in firefox.
That's normal. GStreamer isn't a plug-in. It's used behind the scenes for MP3, MP4 and AAC playback.
> GStreamer is installed on my system in both versions 1.0.5 and 0.10 plus a
> whole bunch of related libraries and GTK plugins.
> Trying to play any of those MP3 files in the nightly build
> (firefox-40.0a1.en-US.linux-x86_64) with a brand new profile fails likewise,
> now the message being "video can't be played because the file is corrupt",
> which means the same thing as the message in the German localization, so
> it's not a localization issue.
I'm sorry to say I don't know where the problem lies then. Someone else is bound to figure it out.

After looking into this a little more I come to the conclusion, that this a specific MP3 issue. At least mpeg-4 (.m4a) audio files play fine.
We saw from Gingerbread Man's comments that there is a connection to the gstreamer library. I find two switches in my preferences (about:config) pertaining to gstreamer: media.gstreamer.enabled and media.gstreamer.enable-blacklist. Both are "true" by default. If I set the latter to "false", everything works fine for me!
There seems to be a connection to Bug 981869 which was deemed to be fixed in v37. But this fix seems to introduce new problems with mp3 audio on openSUSE but also in other contexts (c.f Bug 1133634 Comment 5).

As an interim workaround, is there any way on Linux to force Firefox to hand the stream to VLC or another external player?
There appear to be about:config options for MS Windows ( Direct Show and Windows Media Foundation booleans ) but nothing equivalent I can locate for Linux.
Thanks.

I'm having the same problem on openSUSE 13.1, updated gstreamer, etc... Stopped working after certain Firefox update. No SoundCloud, etc. Chrome can play everything without any problem.
Please, mark as confirmed. I hope someone would be able to locate and fix this bug. I really miss playing mp3 files in browser.

(In reply to Edwin Flores [:eflores] [:edwin] from bug 981869, comment 28)
> [Risks and why]: MP3 playback could stop working for some Linux users
Please explain what these users are supposed to do to get MP3 playback to work again.

(In reply to wf212 from comment #7)
Setting media.gstreamer.enable-blacklist=False finally made it work, thanks a lot! Soundcloud and pro.beatport.com didn't work with Iceweasel 39.0~a2+20150406 (from mozilla.debian.net) on Debian Jessie.
I tested again, downgraded to 38.3.0esr-1~deb8u1 and there it worked also with the default of media.gstreamer.enable-blacklist=True. The I upgraded to 41.0.1-1~bpo80+1 and it continues to work. Whatever caused this issue, now it's resolved for me. Finally I don't need to keep switching to chrome.